I got burned by a helium tank valve. How?

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polypterus
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Hi folks. I had a strange thing happen to me and I was wondering if some of you physics gurus could explain it to me. It's my understanding that gas escaping from a pressurized tank makes things cooler. In fact I recently had to empty my scuba tanks for transport and the valves did indeed ice up as the air escaped. However the other day I found a helium tank it my garage. This is the cheap non-refillable kind you get from toys-r-us for blowing up balloons. It has two valves. A metal one you turn and then a rubber nozzle you put the balloon over and hold sideways to blow up the balloon. My four year old son was standing next to me so I decided to blow up a balloon for him. I turned the metal valve a bit, put a balloon over the nozzle and pushed it sideways. In a couple seconds the thing got so hot it actually burned my thumb and I had a blister for a few days. How could this happen?
 
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There's very little, if any, difference between the effects of extreme heat and extreme cold. That applies to physical sensation as well as cellular disruption. I suspect that you flash-froze your finger and your brain interpreted it as a burn.
 
That crossed my mind however it really did feel like a burn. In any case I'm tempted to try it again with thermometer handy although I'm not sure how I'd use it. The other thing is my scuba tanks which are under much higher pressure do get cold however I can still lay my hands on the valve without getting injured.
 
At ambient conditions helium has a negative Joule Thomson coefficient so will warm when expanded - hence you burnt yourself. Hydrogen and Neon also have negative Joule Thomson coefficients at room temperatures.
 
Hi hamishnichol,
hamishnichol said:
At ambient conditions helium has a negative Joule Thomson coefficient so will warm when expanded - hence you burnt yourself. Hydrogen and Neon also have negative Joule Thomson coefficients at room temperatures.
The rise in temperature for helium is less than 10 degrees F so there's no chance that can happen.
 
Q_Goest said:
Hi hamishnichol,

The rise in temperature for helium is less than 10 degrees F so there's no chance that can happen.

I'd misread the type of cylinder so assumed it was a 200bar helium cylinder which would give ~ 12K increase in temperature which on a hot day could just about push it from hot to touch to ouch!